This invention relates to dynamoelectric machines such as homopolar generators, whose capability can be much improved by the use of thermosyphons for cooling.
Homopolar generators are a long known type of dynamoelectric machine presently having renewed interest as pulsed power sources with characteristics of producing large magnitude direct currents at low voltage. Briefly and generally, such a machine has a stator with a magnetic core and some field excitation with a rotor that has a central magnetic core for flux path continuity and also one or more highly conductive elements on its surface in which unipolar current is induced during rotation of the rotor by some sort of prime mover. In recent designs the rotor conductor is essentially a cooper shell on the outside of and insulated from the rotor core. On the surface of the copper shell rides a large array of current collection brushes. The temperature rise of the brushes and shell have limited the current pulse interval to seconds or less. Restrictions result from brush life degradation and distortion of the copper shell at elevated temperatures. Various forms of cooling have been used or proposed to try and permit more continuous operation of the machine for greater duration of current pulses. These have so far been largely unsuccessful in substantially increasing the pulse interval. The eventual goal is to get pulses of durations of minutes or tens of minutes in magnitudes of millions of amperes. Much improved cooling techniques are required to do so. As background for homopolar pulse generators with solid brush systems and the attendant pulse interval limitations resulting from thermal effects, reference is made to an article by Taylor et al., entitled "Solid Brush System Evaluation for Pulsed High Current Applications" in Wear, 78 (1982) 151-162, which is herein incorporated by reference.
In accordance with the present invention, an improved method of cooling dynamoelectric machines such as the aforementioned homopolar generators is provided using the high acceleration field of rotation of the rotor to induce circulation in thermosyphons located in the rotor. Thermosyphons are generally known in various types of rotating apparatus such as is described in Advances in Heat Transfer, Vol. 9, Irvine et al., Ed., Academic Press, 1973, article titled "Advances in Thermosyphon Technology" by D. Japikse, pp. 2-106, which is incorporated by reference herein. One or more thermosyphons are used in the rotor of the apparatus to accept heat, such as that generated in the copper shell of the homopolar generator, and to distribute that heat to a cooler radially inward rotor portion. The thermosyphon is a passageway, such as one of a tubular configuration having a circular cross section, which may have a configuration such as that in which a first leg is disposed parallel to the rotor axis in the radially outward portion of the rotor, a second leg is disposed radially outward and a third leg is disposed parallel to the axis in a radially inward portion of the rotor. A fluid coolant in the passageway is influenced by rotation and thermal effects to circulate from the outer portion to the inner portion of the rotor and to return in a closed loop. A number of such thermosyphons can be arranged circumferentially about the rotor so that all portions that need to be cooled, such as the copper shell of the homopolar generator, can be effectively cooled ensuring against distortion of the shell and against undue brush deterioration so as to permit much longer term pulse intervals from the machine.
In further refinements of the invention the passageway of the thermosyphon has a flow divider extending along its length except for the ends of the passageway so that the fluid in its closed loop circulation does not tend to intermix. Additional features include a twist portion of the flow divider so that the maximum use is made of the available buoyant energy that drives the flow. In addition, there is a convection trap, which may be a bump or other nonuniformity on the flow divider, to provide a positive unbalance to the flow channels to assure proper starting of the thermosyphon in operation. These and other aspects of the invention are more fully described hereinafter so that it will be seen that the invention offers marked improvement in the cooling of dynamoelectric machines and can be applied in a variety of ways.